Epithelial Na channels regulate the rate of Na reabsorption by epithelia such as the renal collecting tubule and the distal colon, and therefore serve to maintain salt and fluid volume homeostasis. These channels have been studied at the single-channel level in renal tubules and in cultured kidney cells, but not in the colon. This gap will be filled by investigating both macroscopic and single-channel currents in isolated rat colonocytes using patch-clamp techniques. First, conditions and methods of cell isolation for the study of these currents will be optimized. Conditions will be sought which preserve both the morphological polarity of the cells and the function of the Na channels as assessed by amiloride- sensitive whole-cell currents. Then the degree of similarity in the functional properties of colonic channels compared to those of the kidney will be examined. Much is known about changes of transepithelial transport rates in the colon with development of the animal and with mineralocorticoid status. The behavior of Na channels in the colon will be studied under these conditions to see to what extent these changes reflect changes in channel activity, and how channel activity is modulated under these circumstances. The characteristics of the channels will be monitored at three different stages of post-natal development: suckling (10 days old), weanling (25 days old) and adult. The selectivity, current-voltage relationship, single-channel conductance and kinetics of opening and closing of Na channels will be assessed. These studies will focus on the question of whether the observed changes in transport reflect qualitative differences in channels (conductance, selectivity, open probability) or differences in the number of channels. Finally, the regulation of channels by adrenal corticosteroid hormones will be investigated. These investigations will distinguish between qualitative or quantitative differences in the channels under basal conditions and during stimulation with mineralocorticoids and glucocorticoids. These studies will give insights into the basic mechanisms of colonic ion transport and its regulation, and will help in understanding diseases in which epithelial transport is deranged such as diarrhea and cystic fibrosis.